The production of natural gas or methane from an underground reservoir requires purification of the feedstocks to remove undesired components, for example nitrogen or carbon dioxide. The production of crude oil and natural gas from an underground reservoir can be "enhanced" by the introduction of a gas, such as nitrogen, into the reservoir to increase the pressure. This elevated pressure increases the amount of crude oil that can be recovered from the reservoir. As more crude oil is removed from the reservoir, the concentration of nitrogen in the crude oil increases from the naturally occurring level.
The crude oil recovered from the underground reservoir is flashed to a lower pressure and separated into liquid crude oil and gaseous natural gas streams. The majority of the nitrogen remains in the natural gas stream, and must be separated or "rejected" from the natural gas in one or more "nitrogen rejection units" or NRU.
The NRU comprises precooling and distillation processes. The gaseous natural gas stream is cooled in a precooler, and then separated by distillation into at least two streams, one a substantially methane product stream and the other a substantially nitrogen waste stream. Both the methane and nitrogen streams are countercurrently heat exchanged with the feed in the precooler to cool the natural gas feed stream to the distillation column.
Previously, the feed to a Nitrogen Rejection Unit (NRU) was natural gas with the naturally occurring nitrogen content and thus the feed to the NRU contained a constant nitrogen feed composition. Recent methods of enhanced oil recovery utilizing nitrogen injection/rejection processes necessitated the design of a NRU that will process a feed of widely varying composition. Conventional heat exchanger designs for an NRU with variable nitrogen content in the feed currently have large inefficiencies in heat transfer caused by the requirement to oversize the heat exchange circuits to accommodate changes in coolant flow with time. Oversize circuits reduce the heat transfer coefficients between major duty streams. The nitrogen flowrate may increase by a factor of 160, coupled with a methane flowrate decreasing by a factor of 5. These oversized heat exchangers use a fixed plate-fin configuration with constant passage number and passage arrangement of each circuit throughout the life of the particular project.
These inefficiencies of heat transfer cause heat exchanger designs to be much larger and more costly than would be necessary for a design with a fixed feed composition. There exists a need to decrease heat exchanger size and cost by improving the heat transfer process.
Enhanced oil recovery with nitrogen injection/rejection has been commercialized on a large scale only recently. Attempts have not been made to solve the heat exchanger inefficiency problem, but rather only to overcome the problem by installing more heat transfer surface.